1. Technical Field
The invention introduces an extreme low formaldehyde emission urea-formaldehyde (UF) resin with a novel structure and its preparation method which belongs to the timber processing industry.
2. Description of Related Art
In 2009 Chinese artificial board production is about 115,466,500 m3. China has become the biggest artificial boards producing country, and also its wood adhesives yield, especially the UF resin yield is the biggest in the world. In this year, the wood adhesive yield of China was above 8 million ton, and the UF resin was more than 6.5 million ton above 80%. With the economic development and the strength of environmental concern, many countries such as Europe, American, Japan, etc., established more and more strict requirements on the poison gas emission of products, especially the formaldehyde emission of artificial boards. It made a great challenge to the boards manufacturing enterprises in China.
The UF resin has many advantages, such as low cost, abundant raw materials source, good physical performance, fast solidification and colorless glue line. Compared with other wood adhesives, its lowest cost is inapproachable. But a well-known problem of UF resin is its emission of formaldehyde, both from the adhesives and the glued products, which will make great air pollution. Because the products made from UF resin are usually used in interior, it is extremely harmful to human healthy. This problem is getting more and more attention, and many regulations are carried out, so the low formaldehyde released boards must be developed. In 2001 China has set E1, E2 grade for artificial boards in standard Interior decoration material—the limited value of formaldehyde emission from artificial boards and their products (GB18580-2001). So only the environmental and poison less UF resin can satisfy the market requirement both from inner and aboard.
Traditional methods to modify the UF resin are reducing the dissociative formaldehyde content or adding the formaldehyde scavenger. The scavenger will lose its efficiency after a short period, and the unstable structures such as dimethylene ether and hydroxymethyl also exist in the glued products which could decompose and release formaldehyde continuously, so this method could not solve the problem radically. To modify these unstable structures is the essential solution.
There are many methods to modify UF resin, and about dialdehyde monomer used in UF resin to improve its physical and anti-water performance, some researches have been studied. In 1978, William used glyoxal, phosphoric acid and oxalic acid as cross-linking agent to improve the dynamic intensity of UF foam. WO94/04584 synthesized formaldehyde, urea, dialdehyde and some amine compound under acid condition to get some cyclic urea structure to improve the physical performance of UF resin. But the dissociative formaldehyde content of resin is high, and the formaldehyde emission of its board products only achieved E1 quality. In these methods, the unstable structures to release formaldehyde also existed. So we should synthesis UF resin with low formaldehyde release and good performance form the structure design.